Modern cellular telephones use single ended front ends. Specifically, the power amplifier (PA), the duplexer, and the low noise amplifier (LNA) are all single ended. Often this architecture is propagated further down the chain, through the first frequency converters. In the circuitry that follows the converters, the phone transitions to differential.
The original designs all used discrete transistors that are very difficult to and expensive to configure differentially. The best performance at the onset of a new technology is typically through the use of discrete components, and the original requirements called for state of the art performance. At some point when the technology has advanced sufficiently that it is no longer performance driven, circuit integration becomes an option.
FIG. 1 illustrates a functional block diagram corresponding to cellular handset of the prior art. The antenna is connected to a single ended transmit path and a single ended receive path. Both paths are connected to the rest of the radio which would include the IF, the baseband, the processor, and the software.
The antenna is inherently differential. The ground plane of the printed circuit board acts as a reflector and ground return as is the second pole of the dipole. Nonetheless, the antenna is perforce driven as though single ended. This topology has several problems. The grounding of the both the LNA and the PA must be minimized, often at significant cost. The return current of the antenna gets distributed throughout the phone, causing interference. The PA itself broadcasts, requiring shielding within the phone. Any interference will be picked up by any single ended amplifier.